1. Field of the Invention
The present invention relates to an optical bandpass filter having two pass bands.
2. Description of the Related Art
In the field of optical communication for example, an optical filter is used to separate signal light of a specific wavelength from light of any other wavelengths (e.g., noise light). Regarding the conventional optical band-pass filters of this type, there is known an exemplary one where a dielectric multilayer film is laminated on a transparent glass plate. In this case, normally the laminated structure of the film is so set as to have a single pass band.
In wavelength division multiplex communications where a plurality of signal light outputs of mutually different wavelengths are transmitted through multiplexing, there may arise the necessity of employing an optical band-pass filter having two pass bands. Some attempts have been made heretofore to obtain two pass bands by optimizing the laminated structure of a dielectric multilayer film. In this case, however, there exists a problem that it is impossible to set two pass bands independently of each other when setting the pass bands in compliance with variations of the signal light wavelengths. More specifically, since a plurality of multiplexed signal light outputs are incident upon an optical band-pass filter via the same optical path, the other pass band also changes depending on any change of one pass band at the time of fine adjustment of either pass band performed by changing the angle of incidence upon the dielectric multilayer film.